1. Field of the Invention
The present invention relates to an AM radio receiving circuit.
2. Description of the Related Art
An AM radio receiving circuit generally removes pulse noise included in a received AM modulation signal. For example, Japanese Patent Application Laid-Open Publication No. 2001-186031 discloses an AM radio receiving circuit that removes pulse noise after a detecting circuit converts an intermediate frequency signal to an audio signal.
FIG. 7 shows a typical configuration example of an AM radio receiving circuit that removes pulse noise after detection. An AM radio receiving circuit 100 is constituted by including a high-frequency amplifying circuit 111, a frequency converter circuit 112, a high band component removing circuit 113, an intermediate-frequency amplifying circuit 114, a detecting circuit 115, a pulse detecting circuit 116, a linear interpolating circuit 117, and a low-frequency amplifying circuit 118.
The high-frequency amplifying circuit 111 amplifies an AM modulation signal received through an antenna 120 and outputs the signal. The frequency converter circuit 112 converts an amplified high-frequency signal to an intermediate frequency signal and outputs the signal. The high band component removing circuit 113 removes a high band component included in the intermediate frequency signal and outputs the signal. The intermediate-frequency amplifying circuit 114 amplifies and outputs the intermediate frequency signal.
The detecting circuit 115 takes out and outputs an audio signal from the intermediate frequency signal. The pulse detecting circuit 116 detects and outputs a section including pulse noise based on the intermediate frequency signal output from the frequency converter circuit 112. The linear interpolating circuit 117 performs a linear interpolation process based on the detection result of the pulse detecting circuit 116 for the audio signal output from the detecting circuit 115 to remove noise and outputs the audio signal. The low-frequency amplifying circuit 118 amplifies lower frequencies of the noise-removed audio signal to output sound from a speaker 130.
By the way, when the pulse noise included in the AM modulation signal goes through processes such as the detection, the noise generation section is stretched. FIGS. 8A and 8B show waveforms of an audio signal including the stretched pulse noise and an audio signal after the linear interpolation. As shown in FIG. 8A, if the pulse noise is stretched, a section to be interpolated becomes long. Therefore, if the linear interpolation process is performed for such an audio signal, the processing amount of the interpolation process is increased and the interpolated waveform is distorted as shown in FIG. 8B, which deteriorates the sound quality.